Field of the Invention
This disclosure relates to fabrication of flat panel displays. More particularly, the disclosure relates a fiber laser-based method and system configured to provide a substantially uniform polycrystalline structure by controllably annealing an amorphous silicon backplane with a laser beam from one or more fiber lasers.
Prior Art Discussion
The Flat Panel Display (FPD) fabrication environment is among the world's most competitive and technologically complex. Device designers and manufacturers continually strive to satisfy the worldwide consumer's appetite for higher resolution, brighter, larger displays, low power consumption and faster video capabilities for phones, PDAs and other compact products—all cost than the previous generation of technology.
The thin film transistor (TFT) technology is the basis for the FPD that can be either high-resolution, high-performance liquid crystal display (LCD), as shown in FIG. 1, or organic light emitting diode (OLED) FPD. The TFT display circuits are made on a thin semi-transparent layer of amorphous silicon (“a-silicon or a-Si”) and arranged in a backplane across the layer to correspond to respective pixels.
The industry realized that using poly-silicon (poly-Si), which has the carrier mobility approximately two orders of magnitude greater than that of a-Si, substantially reduces the pixel size, improves the aperture ratio, and pixel resolution. As a result of these properties of poly-Si, portable/mobile electronic devices now feature high resolution flat panel displays.
There are two fundamentally different approaches for converting the a-Si into poly-Si through crystallization (annealing). One is a thermal annealing (TA) approach, and the other is a low-temperature poly-silicon annealing (LTPS) approach, which is of particular interest here. In the latter, a-Si is initially thermally treated to convert into liquid amorphous Si, and then it is maintained in the molten state for a certain period of time. The temperature range sufficient to maintain the molten state is selected to allow the initially formed poly-crystallites to grow and crystallize. The LTPS approach is based on two generic methods—Excimer Laser Annealing (ELA) and sequential lateral solidification (SLS), with the latter being the subject matter of the present disclosure.
In ELA, the excimer laser fires pulsed ultraviolet (UV) beam at 3xx nm wavelength directed at an a-Si film coated upon the substrate. The film is heated and melted at a thickness smaller than the full film thickness. The isolated islands of remaining a-Si on the bottom of the film serve as seeds for further crystal growth. The uniformity of resulting grains, which is the key to high performance of the end device, such as a flat screen, is achieved as a result of multiple irradiations of each location with the same fluence when scanning the film with at least 0.4 mm wide beam. Typically, at least 20 pulses are necessary to obtain the desired grain uniformity at each location of the film.
One of the basic issues driving the development of state of the art ELA is the manipulation of the intrinsically unstable condition of the lateral growth in such a way as to allow for more flexible control of film manufacturing. In this sense, multiple techniques, dealing with this issue, can be grouped together as controlled SLS.
In contrast to the above-discussed traditional ELA, the SLS includes melting the entire thickness of the a-Si film without seed-islands at the bottom of the film by a beam from an excimer laser. As a result, crystallization fronts grow from opposite sides of the molten film. In other words, the growth is lateral. The laterally developed crystalline grains can be elongated to large horizontal dimensions. The latter is advantageous since electron mobility increases as grains grow larger.
The lateral growth is accomplished by shifting the film coated substrate and laser beam relative to one another. The technique including irradiating each location of the film twice is known as a 2-shot SLS.
Historically, excimer lasers, used in both ELA and SLS processes, dominate annealing of TFT flat panel displays. Excimer lasers provide a wide range of processing power, with average range of processing powers up to 300 W and higher, energies higher than 1 J and pulse width typically ranging between 30 and 300 ns. Also, Excimers lasers deliver UV light at the wavelength (308 nm), which is directly absorbed in a-Si without additional frequency conversion.
The pulse frequency of the excimer laser is relatively low. To the best of Applicants' knowledge, it does not exceed 6 kHz in SLS processes and considerably lower in standard ELA. As to the SLS, with KHz frequencies leading to high energies, the excimer requires multiple gas changes over a day-long period of operation which makes it unsuitable for mass production.
The excimer-based annealing system is a large, cumbersome structure (FIGS. 2 and 3) costing anywhere from several hundred thousand to more than 10-15 millions of dollars on the market. In other words, it is expensive. In operation, the excimer is known for low uptime (or, conversely, high downtime) for a variety of reasons including, among others, frequent gas refills and subsequent adjustments. In addition to the latter, high maintenance cost also includes expensive and labor-extensive tube replacement once every few months.
A need therefore exists for replacing the excimer laser with a fiber laser source which is simple, inexpensive and requires a minimal or no maintenance at all.